Zero-Gap Bipolar Membrane Electrolyzer for Carbon Dioxide Reduction Using Acid-Tolerant Molecular Electrocatalysts

The scaling-up of electrochemical CO2 reduction requires circumventing the CO2 loss as carbonates under alkaline conditions. Zero-gap cell configurations with a reverse-bias bipolar membrane (BPM) represent a possible solution, but the catalyst layer in direct contact with the acidic environment of a BPM usually leads to H2 evolution dominating. Here we show that using acid-tolerant Ni molecular electrocatalysts selective (>60%) CO2 reduction can be achieved in a zero-gap BPM device using a pure water and CO2 feed. At a higher current density (100 mA cm–2), CO selectivity decreases, but was still >30%, due to reversible product inhibition. This study demonstrates the importance of developing acid-tolerant catalysts for use in large-scale CO2 reduction devices.


Fabrication of gas diffusion electrodes
Catalysts were deposited onto Sigracet 39 BB carbon paper substrates by spray coating (Harder & Steenbeck Evolution with a N2 stream) from a suspension. For RuO2 anode catalyst a loading of 1 mg cm -2 was obtained by adding 9 mg of the nanoparticles to 1 mL H2O and 1 mL isopropyl alcohol and 80 µL of 5% Nafion solution. The solution was sonicated for 30 min and then spray coated onto a 9 cm 2 carbon substrate placed on a hot plate at 95 o C. For Ag cathode catalyst a loading of 1 mg cm -2 was obtained by adding 5 mg of the Ag nanoparticles to 1 mL H2O and 1 mL isopropyl alcohol and 80 µL of 5% Nafion solution. The solution was sonicated for 30 min and then spray coated onto a 5 cm 2 carbon substrate placed on a hot plate at 95 o C. For molecular catalysts, a catalyst loading of 1 mg cm -2 was obtained by adding 5 mg of the catalyst to 1 mL H2O and 1 mL isopropyl alcohol and 80 µL of 5% Nafion solution. After fully dissolving the catalysts, the solution was sonicated for 1 min and then spray coated onto a 5 cm 2 carbon substrate placed on a hot plate at 30 o C.

Electrochemistry
Electrochemical measurements were carried out using a Biologic SP-200 potentiostat. The membraneelectrode assembly was constructed by sandwiching the bipolar membrane between the anode and cathode layers (with the cation exchange layer towards the cathode and the anion exchange layer towards the anode) and 'cold pressing' the layers together between the bipolar plates of the electrolyser cell. The membrane-electrode assembly was assembled in a 5 cm 2 electrolyser from Dioxide Materials. The electrolyser consists of a titanium anode plate with an active area of 9 cm 2 and a stainless-steel cathode plate with an active area of 5 cm 2 , separated by Teflon spacers. The MEA is sandwiched between these plates to provide a zero-gap assembly to which gas is flowed to the cathode and electrolyte is flowed to the anode. The cell was assembled with a torque of 3 Nm.
CO2 was flowed at 20 sccm, first passing through an H2O bubbler at room temperature to humidify the gas before entering the electrolyser. Anolyte (milli-Q H2O) was flowed across the anode at 15 ml/min. Electrochemistry was performed in a 2-electrode configuration under constant current conditions. Before measurement, the cell was pre-conditioned at open circuit, with CO2 and anolyte flowing, for at least 30 min until the cell resistance was stabilized.
For comparison of activity at different current densities, the measurement was conducted in order of increasing currents on the same cell setup, 10 min at each current with a 30 min pause in between each segment with CO2 and anolyte kept flowing. The faradaic efficiency reported is the initial value for each segment. The experiment was conducted in triplicate, and the error bars correspond to 1 standard deviation.

Product detection
Gaseous product in the gas outlet stream was measured by gas chromatography using a Varian CP-4900 MicroGC with a Molsieve 5Å column (10 m) with Ar carrier gas for H2 and CO detection by a thermal conductivity detector. The measurement was carried out at constant column temperature 100 °C, constant pressure 21.7 psi, backflush time 5 s, and injection time 50 -500 ms.

Characterization
Mass spectrometry and CHN elemental analysis was were performed by the University of Liverpool Department of Chemistry analytical services. Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDX) were conducted with a Hitachi SEM S4800 at 20 kV. X-ray Photoelectron Spectroscopy (XPS) measurements were performed on Thermo Scientific K-Alpha Xray Photoelectron Spectrometer using Al Kα source on a 400 x 400 µm 2 area. The survey scans were performed in 0-1200 eV range at 200 eV pass energy and the high-resolution scans were performed in the respective range at 50 eV pass energy. The obtained spectra were analysed using CasaXPS software (version 2.3.17). The spectra were calibrated using F 1s (689.67 eV) or N 1s (400.5 eV) peaks.

Note on selectivity of [Ni(Cyc)] 2+ and its derivatives
We proposed that [Ni(Cyc)] 2+ -based catalysts outperform Ag in our configuration due to their acid tolerance, but we need to address an alternative explanation that [Ni(Cyc)] 2+ itself or the free cyclam ligand behaves akin to a polymer buffer layer which has been reported to increase selectivity. Polymer buffering, as reported by O'Brien et al., 3 makes use of functional groups that can become protonated with the resultant charge hindering cation transport to the cathode. The amine groups of the free cyclam ligand can be readily protonated (pKa's 11.3, 10.2, 1.9 and 1.6) 4 which would buffer local pH in the manner suggested. However there is no free cyclam ligand on the electrode (due to the very large binding constant to Ni II ) 5 and protonation of the amines does not occur following Ni binding to form [Ni II (Cyc)] 2+ . 4 This is to be expected as when coordinated to the Ni centre the N lone pair is not available and it is unable to act as a Lewis base.
If free cyclam had a positive effect on the selectivity towards CO, then the selectivity would increase with electrolysis time as the concentration of free ligand increased due to decomposition. Instead we see the opposite trend with selectivity decreasing with electrolysis time (Figure 4, 5). Additional control experiments (data not shown) with excess cyclam ligand (0.99 mg/cm 2 , or 0.1 mg/cm 2 ) added to [Ni(Cyc)] 2+ loadings (0.1 mg/cm 2 ) showed a decrease in CO selectivity, table S1, confirming that excess free cyclam ligand does not play a role in achieving high CO selectivity.
Instead we highlight that [Ni(Cyc)] 2+ molecular catalysts are known to have a high selectivity for CO production in solution, including at low pH (down to pH 2 has been reported). 2,[6][7][8][9][10][11][12][13] This selectivity has been shown to arise from the high binding constant for CO2 by the Ni I centre, with KCO2 > KH+, giving rise to the high CO:H2 ratios observed in catalysis studies. 7,8 The exact reason for the high KCO2 value is not determined experimentally but it has been proposed that for [Ni(Cyc)] 2+ the CO2 adduct is stabilised by hydrogen bond formation with the N-H groups of the cyclam. 8 With [Ni(CycCOOH)] 2+ we proposed previously that the -COOH group may also have a role in stabilising CO2 adducts and in facilitating their protonation further aiding selectivity. 2 In contrast Ag catalysts are known to preferentially reduced H + with H2 being the dominant product at low pH. 14 Therefore we are confident that the higher CO yields in acid is due to the known high CO2 binding constant of the [

Cyclic voltammetry and quantification of electroactive coverage
Using the Ni 3+/2+ couple which lies more positive of the onset for CO2 reduction and H2 evolution, the electroactive coverage was estimated as 1.5±0.2 × 10 -8 mol cm -2 leading to a maximum TOF of 8±2 s -1 calculated from the highest CO partial current density of 23.2 mA cm -2 . Although some uncertainty remains due to different solvent penetrations from using acetonitrile, this TOF estimate is in line with previously reported TOF on glassy carbon.

Turnover frequency (TOF) calculation
The maximum TOF (s -1 ) was calculated from the CO partial current density, JCO, as follows: where n is 2, the number of electrons per CO2-to-CO reaction, F is Faraday's constant, and Γ is the electroactive coverage of [Ni(Cyc)] 2+ .

Effect of [Ni(Cyc)] 2+ loading
The estimated electroactive content when 1 mg/cm 2 of the Ni catalysts is deposited on the surface is approximately 0.5%. To study the effect of loading we have also carried out performance tests of [Ni(Cyc)] 2+ GDEs with 1/10 and 1/100 loadings (0.1 and 0.01 mg/cm 2 respectively, Figure S4). The 0.01 mg/cm 2 loading gave almost no CO production. However, the 0.1 mg/cm 2 did produce good levels of CO (FE up to 37 %) despite being only 1/10 loading. As it is still underperforming the 1 mg/cm 2 loading used throughout the manuscript we have retained the 1 mg/cm 2 loading in the main manuscript. This results in Figure S4 indicates that although a large proportion of the loaded [Ni(Cyc)] 2+ was not electroactive, a large initial loading by the current method (airbrushing onto a carbon paper substrate) was still necessary to obtain an adequate amount of electroactive catalyst and reach high performance.